A particularly advantageous application of this device consists in measuring velocity profiles of blood flow rates by ultrasonic echography.
The general technical problem to be solved by any ultrasonic echograph destined for measuring and, complementarily, displaying blood flow rates is to eliminate the fixed echoes due to the great reflectivity the biological tissues (in this case the walls of the vessels) which may exceed 40 dB of the reflectivity of blood (red corpuscles). Hence the use of a device for eliminating fixed echoes is indispensable, before any estimation of the velocity profile, to reduce the dynamic of the signals to be treated and to correctly measure the flow rates of anyone section of the vessels, in particular the low velocities which appear in the vicinity of the walls of the vessels and which for that reason are masked by the strong signals coming from the said walls.
A simple solution to this general technical problem consists, for example, in a device for eliminating fixed echoes for an ultrasonic echograph, of introducing a filter, of the delay line type, composed of a line delayed by a recurrence period in parallel on a zero delay line. Weighting coefficients, +1 and -1, respectively, are assigned to the lines which, after weighting, are added up by an adder. So the known filter realizes the difference between two successive echographic lines, which, in principle, leads to a quasi-complete diminishing of the echoes produced by the fixed tissues. However, this technique suffers from the serious disadvantage of also attenuating the signals corresponding to the low flow rates. For example, it may be demonstrated that the response of the above filter as a function of the flow rate is such that, for a recurrence frequency of 1/T=5 kHz, and an emission frequency of N.sub.c =5 MHz, a signal corresponding to V=5 cm/s is attenuated by 30 dB.
This makes the measuring of the flow rates there where they are weakest, i.e. near the vessel walls, difficult, indeed impossible. So it is very important to know the velocities, for example, for the study and the clinical diagnostics of the arteries.
On the other hand it is known that the gradient of the filter mentioned above can be augmented in the gap zone by augmenting its order. Applied to the measuring of the velocities by echography, this comes to no longer treating 2 successive echographic lines but more, say M, by means of a delay line-type filter composed of M parallel delay lines i, respectively (i-1)T. In order to ensure the elimination of the fixed echoes, the M lines are weighted by coefficients the sum of which is zero. All the lines are then added up for being subsequently treated by a velocity estimating circuit, by correlation, phase shifting, etc . . . . Still it is difficult with this type of filter to obtain at the same time a good measure of the studied flow rate profile and a high image rate. If N is the number of lines to be treated (typically N=16 for the peripheral vessels and N=8 for the heart), and if M lines are used for the filter for eliminating fixed echoes, in fact only N-M+1 lines remain which may be used for the velocity estimation, or, for having the same number of lines to be treated, the imaging rate would have to be diminished.
A device for eliminating fixed echoes for ultrasonic echograph is also known from French Patent Application 2 617 982 which corresponds to commonly owned U.S. Pat. No. 4,883,060 in the name of the Applicant and P. R. Pesque comprising, connected in parallel, at least two filters of the same structure combined and shifted in phase among them by (M-1)T, each filter being composed on the one hand of M parallel lines i (i=1, . . . , M), delay lines (i-1)T, respectively, T being the recurrence period of the echograph, on the other hand of weighting means of the M lines i, the sum of the coefficients affected to the lines being zero, and, finally, of an adder of the lines i thus delayed and weighted, due to which device a satisfactory estimation of the low blood flow rates can be obtained, notably in the proximity of the vessel walls, without important diminishing of the lines to be treated, so without reduction of the imaging rate. In this manner two filters are obtained of the order M-1 having in module the same frequency response. So at the output of these filters are available: 2(N-M+1) signals to estimate the velocities instead of N-M+1. These signals are not entirely independent, but with a velocity estimator using a non-downstream linear operation (correlation 1 bit, phase shift, . . . ), an appreciable amelioration is thus obtained. Meanwhile, a correlation finer than the correlation 1 bit would attenuate the amelioration effects thus obtained. The precise technical problem which the present invention proposes to solve consists, while maintaining a high number of lines which may be used for the velocity estimation, of obtaining a filtering, for the rejection of the fixed echoes, which is correct for the low velocities to be measured while remaining near the optimum for the higher velocities.